Synthetic lubricants produced by the oligomerization of alpha-olefins are well-known. Exemplary processes for their production data as early as U.S. Pat. Nos. 2,500,161 of Seger et al. and 2,500,163 of Garwood.
The nature of the alpha-olefin from which these oligomers are produced prescribes the properties of the resultant lubricant.
With few exceptions, the state-of-the-art limits reactants for oligomerization to monomer as above described. While it has been reported that even fairly large amounts of internal and/or branched-chain olefins may be present without extremely adverse effect on the alpha-olefin oligomerization, the resultant lubricants have restricted utility and do not justify their usage as a replacement for naturally occurring petroleum fluids.
In general, linear olefins of from eight to twelve carbons in length (C.sub.8 to C.sub.12) have proven most efficacious. Normal alpha-olefins are generally preferred.
Oligomerization may likewise be accomplished with a wide variety of catalysts. Representative catalyst include such Friedel-Crafts agents as AlCl.sub.3, AlBr.sub.3, BF.sub.3, BCl.sub.3, GaCl.sub.4, and the like. Although each such agent facilitates oligomerization, the activity of the catalyst will differ widely. The very active catalyst such as AlCl.sub.3 will produce extremely high molecular weight polymers in conjunction with utilization of appropriate promoters. Other Friedel-Craft catalysts such as SnCl.sub.4, or GaCl.sub.3 may present disposal problems after use. Moveover, solid catalysts present difficulties with respect to control of the exothermic oligomerization reaction due to the heterogeneous nature of the reaction system.
A preferred catalyst has been boron trifluoride which forms a liquid complex with the necessary promoters and thus lends itself to conventional reaction systems.
Boron trifluoride and the other catalysts must be used in combination with a promoter. The promoter complexes with the BF.sub.3 and in so doing provides an activated system which is required for initiation of the oligomerization reaction. Among the most widely used promoters are the alkanoic and/or inorganic acids which are suitable for selective formation of oligomers ranging from two to four monomeric units.
Conventional practice for conducting the oligomerization reaction has been to admix the promoter with the monomeric olefin in the presence of an imposed atmosphere of BF.sub.3 which is normally gaseous. The presence of excess BF.sub.3 necessary for the reaction is delineated by the observed pressure of BF.sub.3 in the reaction vessel.
The rate of oligomerization to some degree is related to theBF.sub.3 pressure since the probability of excess BF.sub.3 in the liquid reactants is directly related to its pressure.
The imposed pressure of BF.sub.3 can vary over a range of 1 to 4 atmospheres (14.7 psia to 65 psia).
Normally, the reaction temperature is controlled between 20.degree. F. and 120.degree. F. although higher temperatures may be used on occasion. Depending on the temperature and catalyst concentration, reaction times from one-half to twenty hours have been utilized.
The subsequent refining processes can be modified to yield particular product compositions. Where, for example, a lubricant consisting chiefly of higher oligomers is desired, one may remove unreacted monomer and low boiling dimer by distillation at atmospheric pressure. Trimer has also been removed in this manner, but through the less severe conditions of high vacuum distillation.
Various multi-stage oligomerization processes are also known for producing specific lubricant products. These have been limited to produce compositions having properties desirable for particular applications.
For example, U.S. Pat. No. 4,045,507 of Cupples et al. describes a continuous process in which alpha-olefin is first oligomerized in the presence of BF.sub.3 butanol complex to a mixture including trimer and tetramer. That mixture is then continuously withdrawn to one or more other reactors where oligomerization is continued. According to the patent, variation among the process stages may be utilized to obtain increased yields of the trimer at the expense of higher oligomers.
U.S. Pat. No. 4,172,855 of Shubkin et al. describes the production of novel alpha-olefin oligomers by a different multi-stage means. Monomer is first dimerized with a specific catalyst selected from a group of aluminum trialkyls. The dimer is then oligomerized with a different alpha-olefin in the presence of a Friedel-Crafts catalyst such as promoted BF.sub.3. The process is taught to be particularly applicable for the production of controlled copolymers of two or more alpha olefins.
After hydrogenation, the substantially saturated lubricant material is then ready for compounding. Depending upon its composition and properties, it may be employed directly in a wide variety of known applications. Alternatively, known lubricant additives may be incorporated, and/or the material may be mixed with other available lubricants, to achieve the characteristics necessary for given conventional utilities.
Products of many of these known oligomerization reactions have been primarily designed for specialized applications. Aircraft hydraulic or turbine oils, for example, possess low viscosity requirements requiring oligomerization limited to dimers, trimers and tetramers (with emphasis on the trimer). Polymers resulting from such a procedure, however, have limited application as conventional lubricants. Lubricants for higher temperature, e.g., motor oils and industrial lubricants use, require viscosities considerably higher than the aforementioned.